Chromium electroplating



CHROMIUM ELECTROPLATING Henry Brown, Huntington Woods, Mich., assignorto The Udylite Research Corporation, Detroit, Mich., a corporation ofMichigan No Drawing. Application May 7, 1956 Serial No. 582,957

7 Claims. (Cl. 204-51) This invention relates to improvements in theelectrodeposition of chromium from aqueous acidic hexavalent chromiumbaths, and more particularly concerns new exceedingly stable additivesfor such baths which are capable of substantially reducing the formationof spray and mist during electrolysis.

The electrodeposition of chromium from aqueous acidic hexavalentchromium solutions takes place, as is well known, with the evolution ofrelatively large quantities of hydrogen at the cathode and of oxygen andozone at the insoluble anodes. The undesirable spray and mist of chromicacid results from the violent bursting of the multitudinous gas bubblesof high surface energy which are released by the electrolysis. sultingfrom this bubble bursting is of considerable volume because of the highcurrent densities used in the plating, the low efficiency of chromiumdeposition and the use of insoluble anodes. Due tothe corrosiveness andhigh toxicity to the workers of this-spray and mist, and its well knowndeleterious contaminating effect on other plating baths such as nickel,copper, cadmium and zinc, it is necessary in large scale production toemploy powerful ventilation to continuously remove the same as it is vformed. For example, to carry away the mist formed in a 2,000 gallontank installation requires an exhaust of as much as 10,000 cubic feetper minute to enable safe continuous production. The use of powerfulventilation is expensive not only from the standpoint of the initialinstallation cost and upkeep maintenance, but also because of theabstraction of the heated air in winter. Moreover, not all of the sprayand mist is removed from the area of the plating tank even when powerfulventilation is employed and this is especially'true on wide platingtanks. In the usual installation, a certain amount of chromic acid mistdoes contaminate the air, especially when stray air currents pass overthe tank during electrodeposition. From the standpoint of toxicity, themaximum safe concentration of chromium trioxide is now .considered to be0.1 milligram per cubic meter of inhaled. air. In addition to thedisadvantages connected with adequate ventilation, there is the furtherdisadvantage of the loss of chromic acid in the exhausted air which mayamount to about 30% of the chromic acid which is used in theelectrodeposition. The amount of chromic acid thus exhausted, togetherwith that which is carried out on the surface of the articlesthemselves, actually represents more chromic acid than that which isused up in the formation of the chromium coating.

It is, therefore, the principal object of the invention to provide astable additive for an acidc hexavalent chromium bath which greatlyminimizes the formation of spray and mist during electrolysis ofsuchbaths with insoluble or highly polarized anodes.

Another important object of this invention is to provide improved acidichexavalent chromium baths and a method for chromium plating whichsubstantially eliminates the necessity for the use of expensiveventilating procedures currently employed.

2,846,380 Patented Aug. 5, 1958 The problems arising as the result ofthe formation of chromic acid spray and mist in commercial chromiumplating, were recognized almost from the very inception of chromiumplating and various attempts have been made to prevent or to greatlyminimize the formation of this spray and mist. As examples of attemptedsolutions, it has been suggested to form blankets on the surface of thebath by the use of various oils, floating objects such as plasticpieces, and various Wetting agents especially non-ionic wetting agents.None of these attempts have been found to be eminently satisfactory, andall have left much to be desired. For example, non-ionic wetting agentsare rapidly oxidized at the anode, and floating plastic pieces are tooreadily displaced when articles are placed in and taken out of the bath.

It has now been found that the highly surface-active compounds of thisinvention, as exemplified by the compounds of Table I, which carry afluorinated carbon chain and a sulfonic group are unusually stable eventhough hydrogen is present in the carbon chains attached to the sulfonicradical, and resist oxidation potentials existing during electrolysiswith insoluble anodes in acidic solutions of hexavalent chromium.Heretofore, no compound a and yet be stable to the extremely powerfuloxidizing The spray and mist reconditions existing at the insolubleanodes during the electrodeposition of chromium from such baths.

The fluorocarbon sulfonic compounds of this invention are usable inaqueus acidic hexavalent chromium plating baths of conventionalcomposition and have been found to lower the surface tension of suchbaths to amazingly low values and cause the formation of small asopposed to large bubbles of electrolytically evolved gases.Concurrently, the compounds of this invention apparently impartsufficient surface viscosity to the bubbles which, in combination withtheir low surface energy, prevents their violent collapse at the surfaceof the bath. The fluorocarbon sulfonic compounds of this inventioncanFormula I:

R MSO X where R represents a saturatedfluorocarbon chainincludingstraight, branched and cyclic fluorocarbon chains of 3 to 12 carbonatoms; M is an aliphatic carbon linkage group containing from 1 to 4carbon atoms, said carbon atoms carrying hydrogen, oxygen, nitrogen orcombinations thereof and positioned between the R group and the sulfur.of the sulfonic group, and X is a cation. Chlorine or bromine atoms maybe present in proximity to the fluorine atoms on the carbon chain solong as a minor number of chlorine or bromine atoms are present relativeto the fluorine atoms, for example, less than about 30% of the fluorineatoms. In this case M may contain hydrogen atoms in addition to chlorineor bromine atoms, and moreover the terminal carbon atom, or otherwisestated the carbon atom farthest removed from the sulfonic group, maycarry a chlorine or bromine atom. Such chloro or bromo substitutedcompounds are still stable to the most powerful anodic oxidation. Thecation X may represent hydrogen or may be a metallic ion from the groupsincluding the alkali metals, alkaline earth metals, rare earth metalsand heavy metals, such as NH Mg, Zn, Ca, Cr, Al, Ni, Cu, Ce, etc. Thesecompoundscan be considered to be a special type of anionic surfaceactiveagent, and in the accomplishment of the ,objectsof this invention theanionic radical constitutes the dominant surface-active portion. Incomparison to the anionic radical, the cation is relatively unimportanteven though additional quantities of certain cations have been observedto increase the stability of the foam, such as zinc and copper.

Typical representatives of the compounds covered by Formula I which aresuitable as additives to hexavalent 4 or other lead alloy anodes. Inless than four hours of electrolysis, 1 gram per liter of thesenon-fluorinated sulfonic acids is oxidized away leaving inorganicsulfate as a decomposition product in the bath. The sulfate thuschromium baths and the optimum concentrationsfor use formed alters theusual ratio of chromic acid anhydride in such baths are given in TableI. (CrO to sulfate and thus interferes with the plating TABLE I OptimumNo. R? --M--: SO3- X Gone. in

bath, grams/liter H 1 oraoror -b- -so;- rr, n Nm, nor-4 where 1z=2-11Iii e c.

H t t r (2)-.." CFa(CFi)4 (I N-C--(IJ -S0s- H, K t NH, 0.22

H H 6 C. r r i H (3)... Oman);- -(J-NCI:J -s0:- H, K%NH4, 0.2-2

e G. H H

H t r i (4) omorm- J-N-ot J- -80g- Ex m, 0.1-1.5

A H e c. i F (6)..." OFdOFalr 'CN--G--H.- SO:- H, Kicltl'fil, H

H 311; H e (6)... CFKOFQP --'iJN-C! -S0s H, K, NR4, 0.2:2

I etc. H

As may be seen from a study of Formula I and Table range of the bathunless the excess sulfate is removed by I, the R group contains not lessthan 3 and not more precipitation. It will be apparent that if othercatathan 12 carbon atoms, and M contains from 1 to 4 carlyst radicalsare being employed in the bath with or inbon atoms so that the compoundsof this invention may stead of sulfate, such for example, as fluoride orfluosilibe further characterized as containing from 4 to 16 carcate, theformation of excess sulfate ion will correspondbon atoms per sulfonicgroup. It will be understood ly afiect the chromic acid anhydride ratioto the catalyst that the proportions set forth in Table I represent onlyradical with a comparable decrease in the plating range of optimumconcentrations and that somewhat lower conthe bath. Because of theincreased difliculties of control centrations may beemployed toadvantage and thatconof the bath caused by the breakdown in the bath ofsuch ccntrations up to saturation may be used with goodrenon-fluorinated sulfonic acids, their use is undesirable. suits. Incontrast, the fluorocarbon sulfonic compounds of the The additives ofthis invention can be made from type covered by Formula I when dissolvedin the bath, fluorocarbon carboxylic acids made by the process asamazingly resist oxidation during electrolysis even when disclosed in U.S. Patent 2,519,983. With such carboxylic extremely high potentials areemployed, and even though acids, amides or nitriles as startingcompounds, the varihydrogen is present on some carbon atoms in thecomous members listed in Table I may be synthesized by conpound.ventional methods employed in organic chemistry. For In a largeproportion of commercial chromium platexample, to synthesize #2 of TableI, 1,1 dihydropering, the chromium is applied over an underlayer ofnickel. fluorohexyl amine is obtained by hydrogenation of per- Wherenickel is the underlayer, the fluorocarbon sulfonic fluorocaproic amideor its nitrile, and thereafter reacting compounds of this invention tendto render the underthe 1,1 dihydroamine with chloroacetylchloride andsublying nickel plate less sensitive to passivation by contact sequentlyreplacing the alkyl halide group with the sulwith the chromic acidsolution before current is applied. fonicradical by treatment withsodium sulfite. In forming a chromium layer over nickel, it has beenThese new soluble fluorocarbon sulfonic compounds found to be desirablein some instances to preliminarily of Table I as the acids or theirsalts show an unexpected 50 cathodically gas (at low voltage) the nickelplated degree of surface activity in the hexavalent chromium article asit is immersed in the hexavalent chromium platbaths. For example, theincorporation of a small quanmg bath and before the higherelectroplating voltage is tity of about .2 gram to 1 gram per liter ofthe comapplied. Such mild cathodic gassing is accomplished by poundsdesignated by the numerals (l), (2), (3) or (4) using voltages below theplating voltage for a short period of Table I greatly minimizes andsubstantially prevents of time. One suitable condition for such gassingis a theformation Qffine spray and mist from a chromic acid potential of2 to 3 volts maintained for about 5-30 secelectroplating bath operatingat room temperature. Al- 011(15- though an ordinary non-fluorinatedaliphatic sulfonic acid In c n as th fluorocarbon sulfonio compounds ofhaving a carbon chain containing at least 8 carbon atoms fi flfl If aflllOIOcflfbOn cal'boXylic or an aromatic sulfonic acid having at leastan 8 carbon 7 example, the compounds, perfluorocaproic acid alkyl chainattached to the benzene ring is fairly stable when merely dissolved inacidic hexavalent chromium CF3(CF2)4COOH plating baths, such materialsare rapidly and completely and perfluorocaprylic acid CF (CF C00I-I ortheir oxidized when the plating solution is electrolyzed using salts isadded to chromic acid baths used for the electroinsoluble anodes, suchas lead, lead-tin, lead-antimony deposition of chromium, for example,baths containing :a the ratio of 100:1 of CrO to S there results adarkish or much more dull chromium deposit, that is, there is a markeddecrease in brightness of the deposit which can be overcome only by theuse of an unusual ratio of CrO to S0 for example, 50:1 instead of theusual lOOzl.

While operating conditions for the baths of this invention are notcritical, it is preferred that the operating temperature not exceedabout 55 C.-66 C. because of possible hydrolysis of compounds containingamide linkages, such as Examples 2 and of Table I.

The examples given below set forth formulations of operative chromiumplating baths useful for decorative or engineering purposes. It will beunderstood that other compounds covered by Formula I and exemplified bythe compounds of Table I may be used in these typical formulations inthe place of the particular examples given. Additionally, it is to beunderstood that mixtures of the compounds of this invention may beemployed as well as the single compounds.

Example I 150-250 grams/liter chromic acid (CrO 1.5-3 grams/liter 80.,ion

0.3-4 grams/liter 1,1 dihydroperfluorohexyl sulfonic acid Temperature,20 C.30 C.

Cathode current density-l00-300 amps/sq. ft. (approximately -30 amps/sq.dm.)

In the formulation of Example I, catalysts other than the sulfate ionmay be employed, such for example as the fluoride ion or thefluosilicate ion or mixtures thereof. The proportion of fluoride orfluosilicate ion employed is an amount which provides an equivalentcatalytic effect to that which is provided by the 1.5-3 grams/liter ofthe sulfate ion.

Example II 400 grams/liter chromic acid (CrO 3-4 grams/liter 80,; ion

0.1-3 grams/liter 1,1 dihydroperfluoroctyl sulfonic acid Temperature, 20C.40 C.

Cathode current density--100500 amps/sq. ft.

In forming a chromium coating on underlayers of nickel or copper, it isdesirable to maintain the ratio of CrO to S0,, between about 75:1 and150:1. Where the underlayer is white brass (20% Cu80% Zn) it has beenfound that the ratio of CrO to 80.; may be increased to as high as200:1.

Example 111 400 grams/liter CrO 2-3 grams/liter S0 ion 1.5 grams/literHgsiFg 0.2-2 grams/liter of CF (CF CH NHCOCH SO Na Temperature--20 C.-30C.

Cathode current density 100-400 amps./sq. ft.

Example IV 200-400 grams/liter CrO 2-4 grams/liter S0 anion 0.5-3grams/liter 1,1 dihydroperfluorohexyl sulfonic acid 515 grams/liter zincdichromate or zinc oxide or zinc carbonate Temperature-20 C.50 C.

Cathode current density150-300 amps/sq. ft.

In the solutions of this invention, zinc ions, copper ions and'mixturesof zinc and copper ions may be advantageously used in concentrations ofabout 5-20 grams/liter, and when such quantities are employed thestability of the foam is increased. 7

This application is a continuation-in-part of my prior copendingapplication, Serial No. 334,082, filed January 29, 1953, now abandoned.

What is claimed is:

1. In a process of electrodepositing chromium from aqueous acidichexavalent chromium solutions, the improvement which consists in addingto the solution a compound having the formula R MSO X, where Rrepresents a saturated fluorocarbon chain of 3 to 12 carbon atoms, M isa carbon linkage group containing from 1 to 4 carbon atoms andpositioned between the R group and the sulfur of the sulfonic group, thecarbon atoms in said linkage group carrying at least one elementselected from the group consisting of hydrogen, oxygen and nitrogen and'X is a cation, said compound containing in the R substituent a numberof atoms selected from the group consisting of chlorine and brominewhich number is minor relative to the number of fluorine atoms presentin said R substituent said compound being added in sufficient amount tosubstantially decrease formation of spray and mist.

2. A method of electrodepositing chromium wherein the formation of sprayand mist is substantially decreased which comprises electrolyzing anaqueous acidic hexavalent chromium solution containing at least about.01 gram/ liter of a compound having the formula R MSO X, where Rrepresents a saturated fluorocarbon chain of 3 to 12 carbon atoms, M isa carbon linkage group containing from 1 to 4 carbon atoms andpositioned between the R group and the sulfur of the sulfonic group, thecarbon atoms in said linkage group carrying at least one elementselected from the group consisting of hydrogen, oxygen and nitrogen, andX is a cation, said compound containing in the R substituent a number ofatoms selected from the group consisting of chlorine and bromine whichnumber is minor relative to the number of fluorine atoms present in saidR substituent.

3. A method of electrodepositing chromium over nickel which comprisesthe steps of incorporating in an aqueous acidic hexavalent chromium bathat least about .01 gram/liter of a compound having the formula R MSO X,where R represents a saturated fluorocarbon chain of 3 to 12 carbonatoms, M is a carbon linkage group containing from 1 to 4 carbon atomsand positioned between the R group and the sulfur of the sulfonic group,the carbon atoms in said linkage group carrying at least one elementselected from the group consisting of hydrogen, oxygen and nitrogen, andX is a cation, said compound containing in the R substituent a number ofatoms selected from the group consisting of chlorine and bromine whichnumber is minor relative to the number of fluorine atoms present in saidR substituent immersing a nickel coated article in the said bath,cathodically gassing said article by applying a voltage to the bath lessthan that normally applied for plating, and thereafter electroplatingchromium from said bath.

4. A method of electrodepositing chromium which comprises the steps ofadding to an aqueous acidic hexavalent chromium solution at least about.01 gram/liter of a compound having the formula R MSO X, where Rrepresents a saturated fluorocarbon chain of 3 to 12 carbon atoms, M isa carbon linkage group containing from 1 to 4 carbon atoms andpositioned between the R group and the sulfur of the sulfonic group, thecarbon atoms in said linkage group carrying at least one elementselected from the group consisting of hydrogen, oxygen and nitrogen andX is a cation, said compound containing in the R substituent a number ofatoms selected from the group consisting of chlorine and bromine whichnumber is minor relative to the number of fluorine atoms present in saidR substituent and about 5 to 20 grams/liter of a metallic ion selectedfrom the group consisting of zinc, copper, and mixtures thereof, andelectroplating chromium from the solution.

5. A method of electrodepositing chromium wherein the formation of sprayand mist is substantially decreased, comprising the steps ofelectrolyzing an aqueous acidic hexavalent chromium solution containingat least about .01 gram/liter of a compound having the formula R MSO X,where R represents a saturated fluorocarbon chain of 3 to 12 carbonatoms, M is a carbon linkage group containing-from -1 to 4 carbonatomsand positioned between the R group and the, sulfur oi the sulfonicgroup, the carbon atoms in, said; linkage group carrying at 'least oneelement selected; from the group consisting of hydrogen, oxygen andnitrogen; and, X is a cation said compound containing .inthe Rsubstituent a number of atoms selected from the group consisting ofchlorine and bromine which number isminor relative to the number offluorine atoms present in said R substituent and regulating thethickness of the foam blanket which is formed by passing ammonia vaporsover the said foam blanket.

6. A bath for the electrodeposition of chromium comprising an aqueousacidic, hexavalent chromium solution containing a compound having theformula RFMSOgX, where R represents a saturated fluorocarbon chain of 3to 12 carbon atoms, M isa carbon linkage group containing from 1 to,4carbon atoms and positioned between the R group and the sulfur of thesulfonic group, the carbon atoms in said linkage group carrying at leastone element selected from the group consisting of hydrogen, oxygen, andnitrogen, and X is a cation, said compound containing in the Rsubstituent a number of atoms selected from the group consisting ofchlorine and bromine which number is minor relative to the number offluorine atoms present in said R substituent said compound beingpresent; in sufficient amount tov substantially decrease formation ofspray and mist.

7. A bath for the electrodeposition of chromium comprising an aqueousacidic hexavalent chromium solution containing at least about .01gram/liter of a compound having the formula R MSO X, where R representsa saturated fluorocarhonchain of 3 to 12 carbon atoms, M is a carbonlinkage group containing from 1 to 4 carbon atoms and positioned betweenthe R group and the sulfur of the sulfonic group, the carbon atoms insaid linkage group carrying at least one element selected from the groupconsisting of hydrogen, oxygen, and nitrogen, and X is a cation, saidcompound containing in the R substitueut a number of atoms selected fromthe group consisting of chlorine and bromine which number is minorrelative to the number of fluorine atoms present in said R substituent.

References Cited in the file of this patent UNITED STATES PATENTS2,750,334 Brown June 12, 1956

1. IN A PROCESS OF ELECTRODEPOSITING CHROMIUM FROM AQUEOUS ACIDICHEXAVALENT CHROMIUM SOLUTIONS, THE IMPROVEMENT WHICH CONSISTS IN ADDINGTO THE SOLUTION A COMPOUND HAVING THE FORMULA RFMSO3X, WHERE RFREPRESENTS A SATURATED FLUOROCARBON CHAIN OF 3 TO 12 CARBON ATOMS, M ISA CARBON LINKAGE GROUP CONTAINING FROM 1 TO 4 CARBON ATOMS ANDPOSITIONED BETWEEN THE RF GROUP AND THE SULFUR OF THE SULFONIC GROUP,THE CARBON ATOMS IN SAID LINKAGE GROUP CARRYING AT LEAST ONE ELEMENTSELECTED FROM THE GROUP CONSISTING OF HYDROGEN, OXYGEN AND NITROGEN ANDX IS A CATION, SAID COMPOUND CONTAINING IN THE RF SUBSTITUENT A NUMBEROF ATOMS SELECTED FROM THE GROUP CONSISTING OF CHLORINE AND BROMINEWHICH NUMBER IS MINOR RELATIVE TO THE NUMBER OF FLUORINE ATOMS PRESENTIN SAID RF SUBSTITUENT SAID COMPOUND BEING ADDED IN SUFFICIENT AMOUNT TOSUBSTANTIALLY DECREASE FORMATION OF SPRAY AND MIST.